1. Field of the Invention
The present invention relates to production of photomasks, and more particularly to a method for producing photomasks using digitally controlled laser output, as well as constructions therefor.
2. Description of the Related Art
Photosensitive media find application in a wide variety of industrial and commercial contexts. These include preparation of offset lithography masters, printed circuit boards and silkscreen stencils.
In typical applications, a photosensitive substrate is exposed to a source of illumination that passes through a transparency containing a negative rendition (typically in black-and-white tones) of the image to be printed, thereby exposing the non-image portions of the substrate. Hereinafter, this transparency is referred to as the "mask" or "photomask." The substrate is then developed using conventional or specialized photochemical processes, resulting in a finished article having the desired black-and-white pattern imprinted thereon.
Conventionally, the mask is also prepared photographically. For example, a camera can be employed to record an image onto sheet film coated with a photosensitive silver halide emulsion, and the latter subjected to well-known development and fixation processes. This method, while reliable, suffers from the expense and inconvenience associated with "wet" processes generally. The silver halide emulsion adds further cost to the transparency material.
Offset lithography represents one application in which photomasks are frequently employed. Lithographic plates are produced by selectively modifying the surface characteristics of a blank so as to facilitate image retention and transfer. In one common arrangement, the surface of a blank plate ordinarily repels organic materials, such as ink, but is receptive to an ink-repellent fountain solution. Upon exposure to suitable radiation (generally followed by some type of development process), the affinities of the surface material reverse, with the exposed areas becoming ink-receptive and repellent to fountain solution. Accordingly, one can impress an ink-transfer pattern on the blank by exerting control over the portions of the blank that receive exposure to the actinic radiation. This control is provided by the mask, which is introduced between the blank and the source of radiation.
A similar process can be used to etch a finished printed circuit board from a blank having a conductive layer that is sensitive to radiation. Introduction of a suitably imaged mask between a blank ensures that only the proper portions of the blank receive radiation. Either the exposed or unexposed portions are removed during the etching process.
In screen printing, a printing substance such as ink or a dispersion of toner particles is passed through a partially blocked porous mesh screen to a receiving substrate. The nonimage areas of the screen are blocked with a suitable material impervious to the printing substance in a pattern corresponding to a reversal of the desired image. The ink can be applied to the screen and directed therethrough by pressure and/or electrostatic forces.
The patterned mesh screen, or stencil, is generally prepared using some form of photoresist to selectively furnish the ink-impermeable non-image areas. In one commonly used process, a hydrocolloid (e.g., gelatin or polyvinyl alcohol) that has been treated so as to polymerize upon exposure to illumination is first coated on the blank porous mesh; additional coatings are then applied, as necessary, to avoid pinholes.
The prepared screen is then exposed to a source of illumination that passes through a suitably patterned photomask. In this case, however, the mask contains a positive rendition of the final image, rather than a negative. The exposed screen is transformed into a finished printing screen by "development" (e.g., washing with water jets) to remove the unpolymerized coating material.
One method of preparing the mask comprises projecting the on-film image recorded by a camera onto a piece of sheet film coated with a photosensitive silver halide emulsion, and subjecting the latter to conventional development and fixation processes. This photographic method suffers from the same shortcomings outlined above with respect to wet photographic processes generally.
One can largely avoid these disadvantages by using the spark-erosion recording techniques described in U.S. Pat. Nos. 4,188,880 and 5,217,829. Contact and non-contact spark-imaging techniques offer speed, convenience, and appreciable reduction in the cost of producing each photomask, as well as amenability to computer control. However, for many applications--particularly those requiring highly precise placement of very fine image features--spark imaging proves less than ideal, especially when the final product must be comparable in resolution to that obtainable using traditional photographic techniques.